In a conventional electrostatographic copying or printing process, an electrostatic image is formed on an image member, and toned with charged toner particles. The toner image is then transferred to a receiver and fixed to the receiver by a suitable method, such as by pressurized contact with a heated fuser. One of the variables effecting the speed at which toner images can be produced in an electrostatographic machine is the temperature of the fuser. Generally, the hotter the fuser the faster the toner images on the receiver can be fused. However, the hotter the fuser, the greater the chances that toner will stick to or offset onto the fuser. To prevent the toner particles on the receiver from offsetting onto the fuser, depending on the toner, a lower fusing temperature can be used in a belt or roller fuser, and/or a longer belt can be used in a belt fuser. Alternatively, if the temperature of the fuser must be maintained to properly fuse the toner, the fuser can be coated with a release aid, usually silicone oil. However, silicone oil tends to affect the gloss of the toner on the receiver. It is also difficult to control the amount of silicone oil on the fuser. If too little oil is applied, toner will offset onto the fuser. If too much oil is applied, the presence of oil on the receiver will cause visible differences in the gloss of the final toner image. In addition, some silicone oil vaporizes during fusing and contaminates the subsystems of electrostatographic machines with a particularly negative impact on photoconductive image members (photoconductors) and the corona charger wires.
There are a few approaches disclosed in the background art to prevent toner offset onto the fuser without using silicone oil. One approach is to add release additives to the toner as disclosed, for example, in U.S. Pat. Nos. 4,814,253 and 4,758,506. The addition of release additives to the toner composition can create a problem, however, because release additives not only prevent the toner particles from sticking to the fuser, but they may also prevent the toner particles from sticking to each other and to the receiver, thereby reducing transfer efficiency. This is a particular problem when several layers of toner are transferred over one another, because the release additives in lower toner layers hinder the adhesion of upper toner layers. In addition, some release additives such as silicones, adipates and azealates added to each toner in a multilayer toner image can cumulatively act as plasticizers, that is, they lower the glass transition temperature of the toner binders which causes the toners of the final toner image to be stickier and more likely to stick to or offset onto surfaces a final toner image contacts when it is stacked or stored.
Another approach used to prevent toner offset onto the fuser mechanism is disclosed in Japanese Patent Application No. 48-82007, titled "Color Electrophotographic Process". The method involves the application of a transparent toner non-imagewise to the entire surface of the photoconductor prior to exposing and developing a color toner image on the photoconductor, and then transferring both the color and transparent toners to a receiver. Although the transparent toner may prevent the color toner from offsetting to the fuser, the application of the transparent toner to the entire surface of the photoconductor prior to the formation and development of an electrostatic image on the photoconductor creates several problems. For example, the excessive quantity of toner on the photoconductor makes it more difficult to achieve complete transfer of the toner from the photoconductor to the receiver. Because transfer is so difficult, much toner will remain on the photoconductor, which makes cleaning the photoconductor more difficult. Therefore, the cleaning system must be designed to handle the excess toner loads or the toner will cause a deterioration in the copy quality and shorten the life of the photoconductor. The application and cleaning of such large quantities of toner and increased wear on machine parts make this process expensive. Lastly, because the entire receiver is coated with the transparent toner, the entire surface of the receiver will have the same gloss, which may not be desirable.
Another method to prevent toner offset onto a fuser is the subject of U.S. Pat. No. 3,945,726, titled, "Electrophotographic Fixing Device". This patent discloses a device which applies an "image-separating agents" onto the leading edge of a receiver just prior to contact between the receiver and the fuser rollers. The application of "image-separating agents" by the patented device is disadvantageous, because it only prevents toner offset at the leading edge of the receiver, it may provide uneven gloss on the final toner image, and it does not prevent toner offset if the release temperature from the fuser is increased.
Australian Patent Au-B-91586/82, titled "Application of Protective Coating on a Toner Image" discloses a method for applying an imagewise "protective light-transmitting coating on a toner image", claim 1, to improve the adhesion of the toner to a substrate, because "different types of toner . . . adhere more poorly to the recepient surface than conventional ink." page 1.
It would be desireable to provide a process which allows for an increased release temperature of the toner-bearing receiver from a fuser while preventing toner offset onto the fuser.